[libata] make ata_wq workqueue local to libata-core module

Now that libata-scsi module no longer calls queue_work() directly,
we can localize the use of ata_wq.

drivers/scsi/libata-core.c

@@ -59,7 +59,7 @@ static void ata_set_mode(struct ata_port *ap);
 static int ata_qc_issue_prot(struct ata_queued_cmd *qc);
 
 static unsigned int ata_unique_id = 1;
-struct workqueue_struct *ata_wq = NULL;
+static struct workqueue_struct *ata_wq;
 
 MODULE_AUTHOR("Jeff Garzik");
 MODULE_DESCRIPTION("Library module for ATA devices");

drivers/scsi/libata-scsi.c

@@ -29,7 +29,6 @@
 #include "scsi.h"
 #include "hosts.h"
 #include <linux/libata.h>
-#include <linux/workqueue.h>
 
 #include "libata.h"
 

drivers/scsi/libata.h

@@ -35,8 +35,6 @@ struct ata_scsi_args {
 	void			(*done)(struct scsi_cmnd *);
 };
 
-extern struct workqueue_struct *ata_wq;
-
 /* libata-core.c */
 extern void ata_dev_id_string(struct ata_device *dev, unsigned char *s,
 			      unsigned int ofs, unsigned int len);